The present invention relates to a method for evaluating the processing parameters in the manufacture of semiconductor devices, particularly useful every time there are lithographic processes the characteristics of which are to be evaluated.
In order to maximize the performance of integrated circuits, it is necessary to control the processing parameters, and more specifically to point out those undesirable occurrences and phenomena which may affect the electrical characteristics of the finished product. Accordingly, it is more and more necessary to identify the sources of variability in the processing steps commonly used in the manufacture of integrated circuits, more specifically in the lithographic processes. This means that it is necessary to use measurement methods and test instruments which are capable of singling out the effects of the specific processing parameter from those related to different manufacture parameters. Generally, independently from the specific manufacturing method, there is the need to evaluate the characteristics of the machines used in the semiconductor device industry to point out the effects thereof on the product obtained therefrom.
More specifically, for this evaluation the following measurements are necessary: (a) measurement of the variation in width of the lines created with different lithographic processes (optical, electron-beam, X-ray, etc.) on plane surfaces or on surfaces with peculiar topographies; (b) measurement of the variation of the width of the lines created with lithographic techniques due to the effects of the neighborhood of other structures or to other occurrences (diffraction or proximity effects); (c) measurement of the linearity of the widths of lines replicated or drawn with lithographic methods in the range of values which is most suitable for the specific exposure machine or for the process under analysis; (d) measurement of the uniformity of layers etching used in the electronics industry, more specifically of the effects created by structures located close to the line to be measured (load effect) and of the sub-etching; (e) measurement of the uniformity of the thickness of the layers doped by ion implant and diffusion; and (f) measurement of the accuracy in registration between two levels, employing any registration system capable of replicating or drawing the specific structures used to evaluate the parameters.
In the prior art, processes are well-known which allow the measurement of the line width and of the accuracy in registration of a plurality of masks, by using the measurement of the resistance of the test resistive lines (see, e.g., the article by C. P. Auschnitt et al., "Application of wafer probe technique to the evaluation of projection printers", SPIE, Vol. 334, Optical Microlithography Technology for the Mid-1980s (1982), or the article by I. J. Stemp et al., "Automatic Testing and Analysis of Misregistrations Found in Semiconductor Processing", IEEE Transactions on Electron Devices, Vol. ED-26, No. 4, April 1979). Generally, the prior techniques consist of replicating or drawing figures on a conductive layer with uniform thickness and resistivity so as to create electrical resistors. Then the resistors thereof are calculated individually so as to obtain the required parameters. However, since the processing parameters (line width and misregistration value) are of the second order with respect to the values of the resistance involved, and more specifically depend on the difference of the individual resistance values found, the error committed in measurement can be even very significant.
In the article by K. H. Nicholas et al., "Measurement and Identification of Distortion, Alignment, and Mask Errors in IC Processing" (J. Electrochem. Soc. SOLID-STATE SCIENCE AND TECHNOLOGY, March 1981) a bridge is proposed for the direct measurement of the accuracy in registration of two configurations, which bridge has on two adjacent branches two resistors produced simultaneously by using two superimposed masks or by using a same mask shifted suitably during two exposure steps or appropriately protected, and two reference resistors having a reciprocally equal value but a much smaller resistance than the test resistors. By applying an appropriate voltage by means of a connected measurement device, it is possible to obtain directly the value of the misregistration being seeked. This solution, though it allows to determine the misregistration value by directly measuring the difference in conductivity between the two test resistors, is however affected by some problems. In fact this circuit requires the measurement of the current flowing through an appropriately connected amperometer. However, this instrument adds a resistance, due, e.g., to the points of the instrument, to the connecting wires or to the amperometer itself. This additional contact resistance has an adverse effect on the measurement, reducing its precision. Furthermore, this known bridge, the two test arms of which are obtained by using the abovesaid two masks appropriately aligned with one another, so as to detect registration errors, does not allow the direct evaluation of the other processing parameters, as above indicated.